A MIMO communications system employs multiple antennas at both the transmitter and the receiver. A MIMO system can be used to achieve improved system capacity, signal coverage and higher user data rates. A MIMO system also provides the possibility of very high bandwidth utilization.
A MIMO system comprises NT transmit antennas and NR receive antennas. Multiple signal streams are transmitted from the transmit antennas and received by the receive antennas. Each signal stream in the MIMO system is transmitted in a separate layer. In an NT×NR MIMO system, the number of signal streams or layers, NL, is smaller than or equal to min {NT, NR}. Generally, an MIMO transmitter utilizes precoding techniques to precode transmit signals with a precoding matrix. A precoding matrix is also referred to as a precoder in the present application. Both terms are used interchangeably. Precoding techniques help reduce interference between the layers and improve signal-to-noise ratio at the receiver.
In practice, a set of precoders may be compiled to form a predefined codebook. Based on channel state information (CSI) estimated by a receiver, a suitable precoder can be selected from the codebook to precode transmit signals at a transmitter.
Often, channel state information is derived from received reference signals. For example, demodulation reference signals (DM-RS) or sounding reference signals (SRS) can be used in channel state information estimation. However, DM-RS signals are precoded. The channel state information obtained from a precoded DM-RS reference signal may misrepresent the actual channel conditions of the physical channels. Further, the reference signals may be transmitted too infrequently to allow prompt or timely estimation of channel state information.
When a precoder is selected based on incomplete, inaccurate, or stale channel state information, the precoder may be less effective in cancelling interference between signal streams or in improving the signal-to-interference-plus-noise-ratio at the receiver. The transmit signals may exhibit high interference or high noise level.
Accordingly, techniques are needed to improve precoder selections in a MIMO system when channel state information estimated from certain types of reference signals does not accurately reflect the current channel conditions.